For many applications of remote surveillance and monitoring, gathering detailed information about the surrounding environment is of prime importance. One approach is based on the spectrometric detection of laser induced fluorescence from aerosols and gases delimited in air volumes. Such air volumes can be several metres to several hundreds of metres long at ranges varying from a few tens of metres to a few kilometres in range, using intensified range-gated inelastic Lidar (Light Detection And Ranging). This technology has the capacity to detect single photons. Direct photon counting has the advantage of providing light information free of any electronic noise contribution generated by the sensor, making the sensor sensitivity an absolute optimum from an electronic design point of view.
However, such a photon counting detection approach is applicable only if the sensor electronics are able to count one photon at the time (individual photon collection and read out). This limits this optimum detection technique to very low photonic signal scenarios such as the night surveillance of traces of fluorescing aerosols. This technique is not practical during the day or with dense plumes of fluorescing aerosols. With high photon flux, present state-of-the-art implementations of counting electronics are unable to react quickly enough to discern individual photon arrivals.
Two known approaches are used to perform an optimum surveillance of fluorescing aerosols with intensified range-gated inelastic Lidar. The first approach is to detect simultaneously the collected photonic signals by photon counting and by the classical analog method (as CCD counts), such as by using an apparatus as shown in FIG. 1. An image intensifier 10 is in communication with a charge coupled device (CCD) sensor 18. The image intensifier 10 includes a photocathode 12, a micro-channel plate 14, and a phosphor screen, or phosphor plate, 16. The CCD sensor 18 evaluates the number of collected photons and, based on a statistical threshold, will switch between the two electronic detection methods, accepting a deterioration of the sensitivity when the phenomenology detected produces too many photons for the “photon counting method”.
The second approach is to expand the dynamic range of photon counting applicability, which is equivalent to increasing the statistical threshold between photon counting and analog detection. This second approach either distributes, as a function of photon wavelength, the collected photons over a linear array of single photon detection capable detectors, or it increases the time sampling of the photon detection signal, which is equivalent to slicing in time a range-distributed photonic source (that is, a photonic source, or light source, that is distributed in space or occupies a certain volume in space).
Switching between a photon counting mode and analog detection (and the corresponding reduction in sensitivity) because the photonic signal is too intense is a drawback of known approaches. Distributing the photonic signal as a function of wavelength (1-dimensional expansion) over a linear array of detectors does not sufficiently reduce the amount of photons per detector for day-time surveillance, or when probing a dense plume of fluorescing aerosols to satisfy the statistical threshold for photon counting detection.
Similarly, unless one can implement the equivalent of several time windows (or range gates, or range intervals) sufficiently short to each contain a single photon and that can be juxtaposed in time and read out by the electronics independently, reducing the time window corresponds to rejecting useful signal and does not contribute to increasing the sensitivity of the sensor.
Furthermore, as for spectrally distributed signals, time distributed signals (again a 1-dimensional expansion) do not sufficiently reduce the amount of photons per juxtaposed time window during day time surveillance or when probing a dense plume of aerosol to satisfy the statistical threshold for photon counting.
In addition to the known approaches described above, another photon counting approach includes a Geiger-mode avalanche photodiode. However, an avalanche photodiode cannot be positioned in matrix as easily as a CMOS detector, or reliably achieve a desired clock speed. Therefore they will have a similar problem of sensitivity as a conventional CCD or CMOS imager, where the detection and the circuitry are placed on the same surface. The surface devoted to the circuitry is not sensitive to light so this diminishes the sensitivity of the chip.
It is, therefore, desirable to provide a method and apparatus that reduces or removes at least one drawback of known approaches, such as by operating a sensor always in photon counting mode.